Remember that live (As it happens) speech, even among professional journalists, may seem ungrammatical when written down, yet be understood perfectly by the audience.
As written speech, you would likely change the sentence from this:
And it may have been that wood along the ceiling where the fire started.
It may have been that [particular ...
Both II and III are grammatical and idiomatic. III is preferable stylistically because the initial verb indicates what is noteworthy.
Similarly, A and B are both grammatical, but B, by starting with the action desired as the imperative verb, strikes me as more effective and more natural.
By the way, I do not view any of the participles in these examples as ...
There are three clauses. The main clause (with the verb emphasised) is:
my eyes picked him out obscurely in the anteroom
There are two participle clauses:
Blinking away the brightness of the street outside
talking to another man.
The first describes what the speaker was doing at the time, and the second describes the person in the anteroom.
[Using all of the profane German I had ever heard], I ordered him to come out.
"I had ever heard" is indeed tensed (finite) but it is a subordinate clause, more specifically a relative clause modifying the nominal "profane German". The subordinate clause is said to be 'embedded' because it is a dependent within the larger construction.
It is the ...
Using all of the profane German I had ever heard, I ordered him to come out.
The non-finite clause starts at: "using all of the profane German".
"The main verb has to be: Their main verb is either a to-infinitive , a bare infinitive , an -ed form , or an -ing form :" The main verb for these purposes is defined as the first verb. That is "using" ...